1. Field of the Invention
The present invention relates to sunglasses and, more particularly, to an improved multi-layer lightweight CR-39™, polarized, dielectric-mirrored sunglass lens specifically designed for watermen, to reduce both overall light transmission and ocular photochemical damage, available in either high-contrast blue-light blocking amber or grey coloration.
2. Description of the Background
Quality polarized sunglasses have evolved to the point where they often incorporate numerous layers and coatings all of which combine to provide a particular light transmission profile. The efficacy of each layer affects that of each subsequent layer, and a good design effort often involves the balancing of numerous optical constraints in pursuit of a synergistic result. The most useful balance for outdoor enthusiasts such as watermen is a lens having a patterned absorption profile over a broad spectral range. More specifically, the lens should absorb higher energy light more strongly than lower energy light, e.g., more UV light than blue, more blue than green, etc.
Ultraviolet radiation falls within a range of wavelengths below visible light, generally between 100 and 400 nanometers. Long UVA radiation occurs at wavelengths between 315 and 400 nanometers. UVB radiation occurs between 280 and 315 nanometers. UVC radiation occurs between 200 and 280 nanometers. Wavelengths between 100 and 200 nanometers are known as vacuum UV. Vacuum UV and UVC are the most harmful to humans, but the earth's ozone layer tends to block these types of ultraviolet radiation. Nevertheless, the occurrence of ocular injury from ultraviolet exposure has increased dramatically over the past few years, and this is thought to be a result of ozone layer depletion. Given current efforts to restore the ozone layer, it is optimistically predicted to reach original levels by the year 2050. Others speculate that the developing black markets for ozone-depleting agents such as CFC refrigerant will add further delay. Bergmanson et al., Practicing Preventative Eye Care With UV-Blocking Eye Wear, Contact Lens Spectrum (February 1998).
According to Prevent Blindness America, the American Academy of Ophthalmology, and the American Optometric Association, “Ultraviolet radiation can play a contributory role in the development of various eye disorders including age-related cataract, pterygium (growth of tissue from the white of the eye onto the cornea), cancer of the skin around the eye, photokeratitis (sunburn of the cornea) and corneal degeneration.” Cataracts are a major cause of visual impairment and blindness worldwide. “We've found there is no safe dose of UV-B exposure when it comes to risk of cataract, which means people of all ages, races and both sexes should protect their eyes from sunlight year-round.” Infeld, Karen, Sunlight Poses Universal Cataract Risk, Johns Hopkins Study, http://www.eurekalert.org/releases/jhu-sunposcat.html (1998).
Age-related macular degeneration (AMD) is the leading cause of blind registration in the western world, and its prevalence is likely to rise as a consequence of increasing longevity. Beatty et al., The Role of Oxidative Stress in the Pathogenesis of Age-Related Macular Degeneration, Survey of Ophthalmology, volume 45, no. 2 (September-October 2000). Macular pigment is also believed to limit retinal oxidative damage by absorbing incoming blue light and/or quenching reactive oxygen intermediates. Many putative risk factors for AMD have been linked to the lack of macular pigment, including female gender, lens density, tobacco use, light iris color, and reduced visual sensitivity. The absorbency spectrum of macular pigment peaks at 460 nm (Id at 165), and it has been calculated that carotenoids reduce the amount of blue light incident on the photoreceptors of the fovea by approximately 40%.
The incidence of visible blue light exposure is a contributing cause of AMD. Photochemical retinal injury in monkeys from visible blue light (441 nm) was shown by Ham et al., “Histologic Analysis of Photochemical Lesions Produced in Rhesus Retina by Short-wave-length Light”, Invest Ophthalmol Vis Sci 17:1029–35 (1978). It was found that short-wavelength light resulted in damage to the photoreceptor outer segments, cellular proliferation, and other symptoms which resembled changes seen in AMD. It was reported that the power required to cause such damage was 70 to 1000 times lower for blue light (441.6 nm) than for infrared wavelengths (1064 nm) based on exposure times ranging from 1 to 100 seconds. This was confirmed by Wu et al. who showed that the mechanism of blue light induced cell death is apoptosis. Wu J et al., “Blue Light Induced Apoptosis in Rat Retina”, Eye 13:577–83 (1999). Moreover, the ten-year Beaver Dam Eye Study was recently completed and is reviewed in the Arch Ophthalmology, vol. 122, p. 754–757 (May 2004). This study proves a direct correlation between the incidence of blue light and AMD but no association between UVA and UVB light and AMD.
As the entire population is potentially exposed to sunlight, the odds ratio of 13.6 and 2.19 for high exposure to visible blue light and AMD represent quite robust evidence in support of the sunlight/AMD hypothesis. Consequently, a sunlens that dramatically reduces visible blue light combined with a high degree of UVA and UVB protection will preserve visual function.
The Food and Drug Administration recommends that all sunglasses, prescription or non-prescription, block 99% of UVB and 95% of UVA. Most sunglasses on the market meet these criteria. Indeed, there are sunglasses for outdoor enthusiasts that can achieve 99% of both UVA & B obstruction. A quality lens will also strive to reduce glare, add contrast, and yet maintain color balance all to enhance vision. However, other than by limiting all visible light, known lenses do not specifically limit blue light, and this would be a great advantage in light of the AMD correlation.
The foregoing requires a lens with an optimum transmission profile that filters the different colors in proportion to their ability to damage the tissue of the retina, thereby reducing the risks of macular degeneration while actually improving vision. Presently, a number of advancements in lens technology give significant control over the transmission profile of lenses.
Polarization
It is common to provide polarized lenses in sunglasses to eliminate the horizontal transmission of reflected light through the lenses of the glasses to the eyes of the wearer. The polarizing layer blocks light at certain angles, while allowing light to transmit through select angles. This helps to negate annoying glare reflected off other surfaces such as water, snow, automobile windshields, etc. A polarized filter is produced by stretching a thin sheet of polyvinyl alcohol to align the molecular components in parallel rows. The material is passed through an iodine solution, and the iodine molecules likewise align themselves along the rows of polyvinyl alcohol. The sheet of polyvinyl is then applied to the lens with colored rows of iodine oriented vertically in order to eliminate horizontally reflected light. The sheet of polyvinyl may be applied to a lens in one of two ways: the lamination method or the cast-in mold method. To polarize a glass lens, the lamination method is used whereby the polyvinyl filter is sandwiched between two layers of glass. For plastic lenses, the cast-in mold method is used whereby the polyvinyl filter is placed within the lens mold. Relevant prior art patents might be seen in the Schwartz U.S. Pat. No. 3,838,913 and Archambault U.S. Pat. No. 2,813,459. A significant benefit of polarized lenses is the elimination of glare from reflective surfaces such as water.
Color Filters
Color filters can also provide excellent ultraviolet obstruction properties. For example, U.S. Pat. No. 4,952,046 (SunTiger) discloses an optical lens with an amber filter having selective transmissivity functions. This is the original “Blue-blocker” patent for amber lenses that substantially eliminates ultraviolet radiation shorter than 515 nm. The lens is substantially transparent to wavelengths greater than 636 nm which are most useful for high visual acuity in a bright sunlit environment. Similarly, U.S. Pat. No. 5,400,175 (SunTiger) discloses an amber filter having a cut-on at 550 nm. However, color-differentiation is highly distorted due to the deep orange tint.
Similarly, PhotoProtective Technologies of San Antonio, Tex. produces lenses having Melanin pigment. These are said to eliminate all the UV (thereby reducing the risks of cataracts); reduce the violet and blue light (to reduce the risks of macular degeneration); and reduce glare.
The value of color filtration is further apparent in U.S. Pat. No. 6,145,984 to Farwig, which discloses a color-enhancing polarized lens with a trichroic contrast enhancer that yields a virtually colorless gray to the eye, and yet improves the areas of color saturation, chromatic and luminous contrast, clarity of detail, depth perception, and haze penetration.
It would be medically valuable to provide a lens that likewise eliminates all the UV light, and also reduces visible blue light to its lowest possible level.
Mirror Coatings
Various mirror coatings have been available to the sunglass industry for decades. These mirror coatings can be applied to the front and/or back surface of a lens to further reduce glare and provide protection against infrared rays. Metallic mirrors comprise a layer of metal deposited directly on a glass lens to create the equivalent of a one-way mirror. U.S. Pat. No. 4,070,097 to Gelber, Robert M (1978). However, most metallic oxide coatings have proven to be very susceptible to scratching and wear, especially near salt water. Salt water tends to degrade such coatings over time. In addition, metallic mirror coatings absorb light and generate heat. The more recent advent of dielectric mirror coatings solve some of the above-referenced problems. For one, dielectric coatings reflect light without absorption, thereby avoiding the discomfort of hot glasses. Moreover, dielectric coatings are more durable than metallic oxide coatings, especially in outdoor coastal environments. For example, a dielectric layer having a medium refractive index, e.g., a mixed TiO2 and SiO2 layer, has been used in a rear view mirror. U.S. Pat. No. 5,267,081 to Pein (1993). Similar titanium and quartz dielectric mirror coatings have been applied to glass lenses. In the context of sunglasses, these dielectric mirror coatings of titanium and quartz prevent salt water damage while providing additional reflection of light. (Titanium Oxide, TiO3 being considered a dielectric material featuring dielectric constant k=20–85, see http://www.semiconductorglossary.com.)
U.S. Pat. Nos. 6,077,569 and 5,846,649 to Knapp et al. suggest a plastic sunglass lens coated with an abrasion resistant material and a dielectric material (including silicon dioxide or titanium oxide). The abrasion-resistant coating layer includes a transparent adhesion layer comprised of C, Si, H, O, and/or N which is deposited by ion-assisted plasma deposition. A second dielectric coating layer is deposited, and a thin metallic mirror layer may be interposed between the abrasion-resistant layer and the dielectric materials to enhance reflectivity and color characteristics. However, the prior art does not teach or suggest how to incorporate a polarizing filter, multi-layer dielectric mirror, and a hydrophobic overcoat in a blue-blocking amber or gray tint lens to provide an outstanding spectroscopic profile, especially for a marine environment.
Hydrophobic Coatings
Hydrophobic coatings are known in a more general context for protecting lens surfaces (U.S. Pat. No. 5,417,744 to Ameron) and for contact lenses (U.S. Pat. No. 4,569,858 to Barnes Hind). Hydrophobic coatings are also appropriate near water to protect underlying layers of a lens over time. Hydrophobic coatings are especially good for protecting mirrored lenses as above. For example, U.S. Pat. No. 5,928,718 to Dillon discloses a protective coating for reflective sunglasses incorporating a conventional resin/polymer type coating for protection of the mirror finish against abrasion and smudging.
Rugate Filters
As an alternative or as a supplement to be used in combination with the above, a Rugate filter is an interference coating in which the refractive index varies continuously in the direction perpendicular to the film plane. The addition of a rugate filter to a lens can block visible blue and UV light, as well as infrared and laser energy, while allowing other visible light to pass unimpeded. Rugate filters are wavelength specific filters that have existed for about a decade. Their simple periodic continuous structures offer a much wider set of spectral responses than discrete structures, and they typically exhibit a spectrum with high reflectivity bands. This allows the possibility of making high reflectivity mirrors with very narrow bandwidth. As an example, Rugate notch filters from Barr Associates use refractory metal oxides for edge filters and beamsplitters. Rugate filters are typically formed by a continuous deposition process, it is an easy matter to vary the mixture deposited on the substrate, and thus vary the index of refraction. An overview of Rugate filter technology can be found at Johnson et al., “Introduction to Rugate Filter Technology” SPIE Vol. 2046, p. 88–108 (November 1993), inclusive of how a simple rugate filter is derived from Fourier analysis. This article shows the utility of refractive index profile tailoring and the advantages of using this technology. Other examples can be found in U.S. Pat. No. 5,258,872 “Optical Filter” by W. E. Johnson, et al. and disclosed in U.S. Pat. No. 5,475,531 “Broadband Rugate Filter” by T. D. Rahminow, et al. Unfortunately it is difficult to manufacture rugate filters as they require expensive vacuum deposition techniques called “sputtering.” In the sputtering process it is difficult to accurately control the sputtering of two materials to precisely vary the index of refraction. Other processes such as laser flash evaporation, ion beam assisted deposition, resistive and electron-beam evaporation do not lend themselves to plastic eyeglass lenses and/or require relatively expensive equipment.
It would be greatly advantageous to provide a synergistic combination of UV-absorbing light-weight CR-39™, polarization, and dielectric mirror technology in such a way as to maximize the benefit to watermen. Specifically, it would be advantageous to provide a combination of: a) outer hydrophobic overcoat to protect the lens from seawater and smudging; b) multi-layer dielectric mirror which further reduces light-transmission and glare; and c) two layers of high-contrast ophthalmic CR-39 (plastic) having either a blue-blocking amber-tint or color-discriminating grey tint, d) the layers of CR-39 sandwiching; a cast-in mold polarizing layer, and arranged to provide an unsurpassed light transmission profile optimum for use on the water in which there is 100% absorption of UVA & B light. It would also be advantageous to provide a Rugate filter in place of or as a supplement to the foregoing dielectric mirror to even further reduce the visible blue light as well as infrared and laser energy.